Organic Chemistry 2 · Acyl substitution and derivative choice
tert-Butyl phenylacetate by Steglich esterification
Conditions: DCC, DMAP (cat.), CH2Cl2
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The mechanism, step by step
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Phenylacetic acid protonates DCC
DCC turns the poor OH leaving group of a carboxylic acid into a good one. The acid first gives its proton to a nitrogen of the carbodiimide, making the central carbon electrophilic.
The curved arrows
- A DCC nitrogen lone pair bonds to the carboxylic acid proton.
- The O–H bond pair stays on oxygen as the carboxylate forms.
The carboxylate adds to form the O-acylisourea
The phenylacetate oxygen attacks the central carbon of protonated DCC, and the C=N⁺ π pair moves onto nitrogen. The O-acylisourea is an activated acyl donor.
The curved arrows
- A carboxylate oxygen lone pair forms the O–C bond to the carbodiimide carbon.
- The C=N⁺ π electrons move onto the protonated nitrogen.
DMAP intercepts the O-acylisourea
tert-Butanol is too hindered to attack the O-acylisourea quickly; left alone, it would undergo O→N acyl migration to an unreactive N-acylurea. Catalytic DMAP, a small strong nucleophile, attacks the acyl carbon first.
The curved arrows
- The DMAP ring nitrogen lone pair forms a new N–C bond to the acyl carbon.
- The C=O π electrons move onto oxygen.
The isourea anion leaves
The alkoxide re-forms C=O and the isourea oxygen leaves as an anion. The N-acylpyridinium ion is a very reactive acyl donor that cannot rearrange.
The curved arrows
- The oxygen lone pair rebuilds the carbonyl π bond.
- The C–O bond to the isourea breaks onto its oxygen.
tert-Butanol attacks the acylpyridinium ion
Even the hindered tertiary alcohol adds to the activated acylpyridinium carbonyl. The tetrahedral intermediate carries a positively charged oxonium oxygen.
The curved arrows
- The tert-butanol oxygen lone pair forms the new O–C bond.
- The C=O π pair moves onto oxygen.
DMAP is released
The alkoxide re-forms C=O and neutral DMAP leaves, so the catalyst is regenerated. The protonated ester remains.
The curved arrows
- The oxygen lone pair rebuilds the carbonyl π bond.
- The C–N bond pair returns to the DMAP ring nitrogen.
The isourea anion becomes DCU
The isourea anion takes the oxonium proton on nitrogen: its O⁻ forms C=O while the C=N π pair bonds the proton. This gives dicyclohexylurea (DCU), whose strong C=O pays for the whole coupling.
The curved arrows
- The oxygen lone pair forms the C=O π bond of the urea.
- The C=N π electrons bond that nitrogen to the oxonium proton.
- The O–H pair returns to the ester oxygen.
tert-Butyl phenylacetate
tert-Butyl phenylacetate forms, DMAP is regenerated, and DCU precipitates from dichloromethane. The Steglich method makes tert-butyl esters under neutral conditions where Fischer esterification fails.
What you'll practice
Draw the Steglich esterification of phenylacetic acid with tert-butanol, showing DMAP intercepting the O-acylisourea and DCU forming at the end.
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